The most common type of dot matrix printer uses a printhead that traverses the width of a sheet of paper in printing a line of text; the printhead incorporates a single column of print elements and printing is effected in a series of incremenetal movements each corresponding to the width of one dot. Examples of such column-sequential dot matrix printers are Zenner et al U.S. Pat. Nos. 3,670,861 and 3,729,070. Another type of dot matrix printer is a line printer that includes a series of N print rods or other print elements aligned along a dot print line, with the print elements displaced one or more character width spaces from each other. In such a line printer, the print elements are shifted incrementally along the dot print line through one or more character width spaces, each increment of movement being approximately equal to the width of a dot, to print the uppermost row of dots in a complete line of text; the paper is then advanced by a distance equal to one dot height and the next row of dots in the same line of text is printed, and so on until the complete line of text has been reproduced. Dot matrix line printers of this kind are disclosed in Howard U.S. Pat. No. 3,802,544 and in Zenner U.S. Pat. No. 4,248,147. The present invention is primarily concerned with dot matrix line printers, but some of the features of the present invention can also be applied to column-sequential printers.
One effective driver mechanism for a print rod or similar dot matrix impact print element utilizes an electromagnet and a permanent magnet in combination in a shared magnetic circuit. In a device of this kind, the armature of the electromagnet is spring biased from an attracted position immediately adjacent the pole or poles of the magnetic circuit toward a released position substantially separated from the magnet poles; usually, the attracted position is the non-print position and the extended position is the print position for the device. The permanent magnet is used to hold the armature in its attracted position and the electromagnet coil is energized to overcome the magnetic force of attraction exerted by the permanent magnet, releasing the armature to move to its print position and drive a print rod or other print element through a printing movement. Examples of this particular kind of permanent magnet/electromagnet driver mechanism, as used in dot matrix printers, are disclosed in Brumbaugh et al U.S. Pat. No. 3,672,482 and Barrus et al U.S. Pat. No. 4,233,894.
This type of print element driver is capable of high speed operation, but frequently exhibits some undesirable attributes. Thus, the permanent magnet is usually connected in series in the magnetic circuit of the electromagnet, as shown in both of the aforementioned patents. With this arrangement, the high reluctance of the permanent magnet materially increases the number of ampere turns that must be developed by the electromagnet coil in order to overcome the bias force exerted by the permanent magnet, so that the device is inherently inefficient with respect to energy consumption. As a consequence, devices of this kind tend to require inordinate levels of energization and may run hot. This difficulty can be overcome by changing the magnetic circuit so that the permanent magnet is connected in parallel with the electromagnet as in Luo et al U.S. Pat. No. 4,273,039. On the other hand, that arrangement usually requires energization of the electromagnet in opposite polarities in order to afford effective operation, increasing the complexity of the energizing circuits for the electromagnet and reducing the efficiency of the device with respect to energy consumption.
In electromagnetic print element drivers that utilize permanent magnets in combination with electromagnets, the armatures have frequently been mounted upon cantilever springs; the cantilever spring serves as a support for the armature of the device and also provides the biasing force that drives the armature and its associated print element from the attracted non-print position to the released print position. A spring of this type normally has a straight line characteristic, whereas in a typical magnet the force exerted on the armature increases inversely as the square of the length of the armature air gap. Consequently, the device fails to utilize the full available force of the permanent magnet in attracting the armature to its initial non-print position. Furthermore, the armature usually terminates its movement to attracted position with substantial impact and with a tendency toward undesirable vibration and secondary motions, since the attractive force of the permanent magnet continues to increase as the armature approaches its attracted position.
In a dot matrix line printer, friction and inertia present major difficulties with respect to movements of the print elements. For high speed printing operations, the movements required of all of the print elements, reciprocating along a dot print line in the printing of successive rows of dot elements, make any increment of added weight and any friction in the drive mechanism highly critical. Conventional reciprocating support structures for the ends of the print rods or like print elements at the print station impose relatively severe limitations on the print rate and frequently lead to inaccuracies in the printed characters, as by misalignment of the columns of dots in the reproduced characters and like effects. On the other hand, precision feeding of the paper is essential to good print quality in a dot matrix line printer, and precise control of the tiny (dot height) increments of paper movement that are essential to a printer of this kind has been extremely difficult to obtain in a line printer operated at a high print rate.